Magnetically shielded magnetic sensor with squid and ground plane

ABSTRACT

A coupling structure for coupling a feedback signal to a superconducting quantum interference device (SQUID) by mutual inductance. In one embodiment the SQUID loop (A) is shielded from external magnetic fields perpendicular to it by a superconducting ground plane (B) at all points except for the pick-up loop (C). A feedback signal is coupled to the SQUID loop (A) by a feedback loop (D) which has a mutual inductance with the SQUID loop (A). During operation, the feedback loop (D) conducts a current in only one direction around the SQUID loop (A). This geometry ensures that the SQUID loop (A) is shielded from external magnetic fields, except at the pick-up loop (C), by the ground plane, and is balanced against fields parallel to the ground plane. The magnetic field produced by the current in the feedback coil (D) is small far from the SQUID (A). The feedback loop (D) is connected to exterior feedback electronics.

FIELD OF THE INVENTION

This invention relates to an improved coupling structure for asuperconducting quantum interference device (SQUID) incorporating a fluxpick-up loop. More particularly, it relates to a nearly planararrangement of coupling structure, SQUID, and ground plane which isself-shielding.

BACKGROUND

Superconducting quantum interference devices (SQUIDs) are extremelysensitive detectors of magnetic flux. When paired with appropriatefeedback and readout electronics, SQUIDs can detect magnetic fieldscorresponding to fractions of a flux quantum (Φ₀).

Because of the nature of magnetic fields, SQUIDs can be used inapplications where light does not penetrate and sound is distorted.SQUIDs can be used to detect underwater objects such as mines andsubmarines, or to determine probable locations of oil and mineraldeposits. They can detect magnetic signals produced by the body as well,detecting the firing of neurons in the is brain inmagnetoencephalography (MEG), or disease in soft tissues in magneticresonance imaging (MRI).

In certain cases, however, this extreme sensitivity to magnetic fieldscan be detrimental to a SQUID's performance in an application. Forexample, a large background field can mask a smaller field that is ofinterest. In this case the SQUID must be very sensitive to see the smalltarget signal. However, if the large background field penetrates even asmall portion of the SQUID's field of view it can swamp the signal.

Typically in such uses of SQUIDs as magnetometers, both the SQUID andthe object under study are located within a magnetic shield. This can bea "shielded room" which is available commercially and which is simply aroom built to reject any external magnetic or s electromagnetic signals.Another option is to completely enclose the object and sensor in asuperconducting enclosure. Since superconductors are perfect diamagnets,no magnetic field can penetrate a superconducting plate or box. (Undercertain conditions, magnetic flux can penetrate a superconductor.However, it is easy to predict the magnetic field strength which will beshielded by any superconducting shield, and to design the shield toaccomplish this task.) Unfortunately, this shielding is not possible inall situations.

One example of such a situation is the use of SQUIDs for magneticmicroscopy and non-destructive testing (NDT), or evaluation (NDE). Inthese applications, spatial resolution is very important. The change ina magnetic signature over regions a few micrometers in diameter can beimportant for pathologists looking at a biopsy sample or for aircraftmaintenance engineers looking for an incipient crack in a corroded weld.The small field of view and the typically small changes in magneticfield that must be detected require the use of extremely sensitiveSQUIDs. At the same time, the small field of view or the fineness of thearray precludes the use of external shields to block background magneticfields from equipment like computers or from the earth itself. Thecombination of very sensitive detectors and an unshielded environmentplaces stringent requirements on the magnetic sensing system.

DISCUSSION OF THE ART

In order to operate SQUIDs as flux sensors, it is usually necessary tohave a means for applying magnetic flux feedback, possibly in additionto a small alternating modulation flux, in order to implement theflux-locked loop. This requires a mutual inductance between a coil (thefeedback coil) and the SQUID loop. The mutual inductance is defined asthe flux introduced into the loop per unit current in the coil. When theSQUID encounters a magnetic field, the resulting change in flux in theSQUID loop causes the electrical output of the SQUID to change. Thefeedback electronics counteracts this change by introducing into thefeedback coil a current which produces in the SQUID loop a flux equaland opposite to that produced by the applied magnetic field. Bymonitoring the current required to stabilize the flux in the SQUID, thereadout electronics measures the magnitude of the magnetic fieldencountered.

The thin-film structures that are generally used for the SQUID andfeedback coils have a disadvantage in applications like a magneticmicroscope, where it is desirable to prevent the SQUID loop fromcoupling to an external magnetic field except at a specially definedlocation. The usual structures expose some of the area of the SQUID loopto external fields, which makes the microscope sensitive to field atplaces other than where sensitivity is desired.

FIG. 1 shows an example of a prior art integrated SQUID/feedback coilstructure used for magnetic microscopy. The feedback coil is very closeto the SQUID loop for maximum coupling and reduced area. Unfortunately,the geometry of this coupling scheme makes the sensor extremelyvulnerable to external magnetic fields.

SUMMARY OF THE INVENTION

The present invention provides a solution to this dilemma. A new SQUIDdesign is proposed wherein the feedback coil and SQUID loop areessentially planar and shielded from external magnetic fields by asuperconducting ground plane.

In this design, the SQUID loop is a ground-planed stripline ormicrostripline structure which is shielded from external fields appliedperpendicular to the thin film, and which is balanced against externalfields in the plane of the film.

More features and advantages of the proposed structure can be understoodwith reference to the drawing figures and the more detailed descriptionof the preferred embodiments. Because of the relative size of thevarious features important to the SQUID sensor's operation, FIGS. 1, 2,and 3 are not to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a prior art SQUID sensor.

FIG. 2 shows a perspective view of the improved SQUID sensor of theinvention.

FIG. 3a shows a cross-section taken through the SQUID loop, while FIG.3b shows a cross-section taken through the pick-up loop.

FIG. 4 shows an equivalent circuit of the SQUID sensor.

FIGS. 5a and 5b show a comparison of the current-voltage (I-V)characteristics of the prior art and improved SQUID sensors. FIG. 5ashows the I-V characteristic of the prior art SQUID sensor of FIG. 1.FIG. 5b shows the I-V characteristic of the improved SQUID sensor ofFIG. 2.

FIGS. 6a and 6b show a comparison of the voltage-flux (V-Φ)characteristics of the prior art and improved SQUID sensors. FIG. 6ashows the V-Φ characteristic of the prior art SQUID sensor of FIG. 1.FIG. 6b shows the V-Φ characteristic of the improved SQUID sensor ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A SQUID detects magnetic fields by counting the number of flux quantathat are applied to the SQUID loop. In all conductors, magnetic fieldsand currents are related. A current flowing in a diamagnetic conductorproduces a magnetic field, and a magnetic field induces a current in anearby diamagnetic conductor. The induced current flows in a directionwhich produces a magnetic field opposed to the original magnetic field,so that the resulting magnetic field is lowered. For most materials atmost temperatures the response of the material is imperfect, that is,the induced current decays and the resulting magnetic field regains itsfull value a short time after the initial current is applied.Superconductors, however, are perfect diamagnets, so a perfect ring ofsuperconductor will not allow the magnetic flux inside the ring tochange at all. This is a thermodynamic property of superconductors. Whenexposed to a magnetic field, a supercurrent is induced in the ring, andthe supercurrent exactly counters the magnetic field to which it isexposed. Thus, in principle, measuring the current flowing in thesuperconducting ring gives the magnetic field strength near thesuperconductor.

Superconductors have limits, as do all physical things. Superconductorsonly superconduct below a critical, or transition, temperature T_(c).They also "go normal" or lose their superconducting properties when thecurrent flowing through them exceeds a critical current, I_(c) ; this isoften expressed at the critical current density (J_(c)) of the material.Finally, superconductors can repel only so much magnetic flux beforeundergoing the transition back to normal materials. The maximum magneticfield a superconductor can take before this transition is called thecritical field, H_(c). These three properties of a superconductor arerelated. At temperatures close to T_(c) or in a high magnetic field asuperconductor can carry less current than its theoretical maximum, andso forth.

Commercial SQUIDs take advantage of these limits. They incorporate oneor two weak-link junctions (labeled "E" in FIGS. 1 and 2) into the SQUIDloop (A). These junctions (E) may be any kind of weak link, and theirnature and number will depend on the particular circuit design. Iffabricated using niobium technology, they would be tunneling Josephsonjunctions or superconductor-normal-superconductor (SNS) junctions. TheNb tunnel junctions also require a pair of shunt resistors. Forhigh-temperature superconductive (HTS) applications, the junctions maybe SNS, step-edge, grain boundary, or thinned weak links. Such junctionsare referred to as "weak links" because they are the locations mostlikely to go normal. Similarly, the SQUIDs may be fabricated from anysuperconductor, as long as the operating temperature of the sensor iskept below the superconducting transition temperature of the materialchosen. Appropriate materials include both conventional superconductorsand high-temperature superconductors. Conventional superconductors areusually metallic and have superconducting transition temperatures belowabout 23K. They include all of the elemental superconductors, theChevrel phases, NbN, Nb₃ Ge, Nb₃ Sn, and other A15 compounds.High-temperature superconductors have superconducting transitiontemperatures above about 23K and as high as about 133K. They are alsoknown as oxide superconductors, cuprate superconductors, and perovskitesuperconductors because of their compositions and crystal structures.These include any of the superconducting phases of LaCuO, LaBaCuO,LaSrCuO, TlBaCaCuO, BiSrCaCuO, TlPbSrCaCuO, HgBaCaCuO and ErBa₂ Cu₃ O₇-δ, in addition to YBa₂ Cu₃ O₇ -δ.

When exposed to a magnetic field, the current in the SQUID loopincreases and decreases periodically as the field increases, with aperiod of one flux quantum. When this happens, there is a periodic riseand fall in the voltage that develops across the SQUID in response tothe constant bias current applied by the control electronics. Bymeasuring the size of the current that has to be applied to the feedbackcoil in order to keep the voltage developed across the SQUID constant,or alternatively by counting the oscillations of the voltage, themagnetic field strength can be calculated.

In order to accurately transform the applied feedback current to ameasured magnetic field the coupling between the feedback coil (D) andthe SQUID loop (A) must be well defined. The feedback coil (D) isnormally coupled inductively to the SQUID loop (A). As seen in FIG. 1,this is often accomplished by using a multi-turn coil over a secondaryhole in the SQUID.

Referring to FIG. 1, a magnetic sensor for non-destructivetesting/evaluation (NDT/NDE) or microscopy consists of a SQUID (J) and afeedback coil (D). The SQUID (J) has a SQUID loop (A), part of which iscalled a pick-up loop (C), and one or two weak-link or Josephsonjunctions (E). In order to obtain strong coupling between the feedbackcoil and the SQUID loop, they must be close together. In thisconfiguration the pan of the SQUID loop (A) coupled to the feedback coil(D) can act as a secondary pick-up loop, coupling external magneticfields into the SQUID.

This secondary coupling causes two problems. The first is a loss ofresolution. The position of the origin of a magnetic signal isdetermined by the position of the SQUID pick-up loop (C). A magneticsignal coupled to the SQUID (J) by the feedback structure may have adifferent origin, in which case the mapping of magnetic signal tophysical location will be distorted. A second problem is a reduction insignal to noise ratio (SNR). Because the desired signal comes from asmall area to increase spatial resolution, the SQUID must be verysensitive. If the feedback structure is much larger than the pick-uploop it may couple more strongly to an extraneous background signal,such as the earth's magnetic field. This spurious signal may be of thesame order as the desired signal, or may be even stronger. In this case,the SQUID may be overloaded by the spurious signal.

The improved SQUID coupling structure of the instant invention is shownschematically in FIG. 2. Here, in addition to the SQUID loop (A) withpick-up loop (C) and coupling loop (D) to feedback electronics (notshown), there is a superconducting Found plane (B). Because asuperconductor is a perfect diamagnet, magnetic fields cannot penetratea superconducting ground plane. This is illustrated schematically in thecross-sections of FIGS. 3a and 3b. FIG. 3a is a cross-section takenthrough the body of the SQUID along line 1--1 of FIG. 2. The groundplane (B) excludes the magnetic fields (H) from the SQUID loop (A) andthe feedback loop (D). This view also shows the dielectric insulation(I) that may be present in an actual structure. FIG. 3b shows across-section taken along line 2--2 of FIG. 2, through the pick-up loop(C). Here there is no shielding, and the magnetic field (H) penetratesthe opening in the pick-up loop (C) where it is detected by the SQUID.Thus magnetic fields (H) perpendicular to the ground plane and to theplane of the SQUID can penetrate only in the area of the pick-up loop.This limits the field of view to the desired area of the plane. Further,because the SQUID loop is entirely planar, magnetic fields in the planeof the ground plane do not introduce any flux into the SQUID. Electricalconnections are made to the SQUID loop at the counterelectrode ofjunctions (E) and via its connection to the ground plane (F).

A further advantage of the improved coupling structure is that thesample being measured is less disturbed by the magnetic field producedby the feedback loop (D) than it is by the field of feedback coils ofconventional SQUIDs. This advantage results from the bifilar geometry ofthe feedback coil and from the proximity of the ground plane, which giverise to a smaller leakage of field than does the conventional spiralcoil.

Since the improved sensor is shielded by the superconducting groundplane from external magnetic fields perpendicular to its plane there areno components of magnetic field that can couple to the SQUID except inthe region of the pick-up loop, that is, the region of interest.

If desired, a second ground plane (G), shown lifted off the structure inFIG. 2, can be added to improve the shielding of the SQUID loop (A) andthe feedback coil (D). The resulting structure is an example of astripline, rather than a microstripline, configuration. Another optionaladdition to the basic scheme is a bias connection (F) between the groundplane (B) in FIG. 2, and the SQUID loop (A). These additional featuresmay enhance the performance of the magnetic sensor, but are notnecessary for the successful operation of the improved coupling scheme.

FIG. 4 shows an equivalent circuit of the improved SQUID sensor. Thepick-up loop (C) lies partially outside the region shielded by theground plane (B) and is exposed to a magnetic flux. The SQUID junctions(E) are shielded from all other fields. A feedback loop (D) isinductively coupled to the SQUID loop (A) to allow for flux-locked loopoperation. During operation, a field is applied to the SQUID using thefeedback coil to counteract the effect of the detected magnetic flux.The feedback electronics monitors the amount of current applied andconverts the current to a magnetic field measurement.

FIGS. 5a, 5b, 6a and 6b show performance data of the prior art andimproved SQUID sensors. An ideal SQUID has an I-V characteristic(compare FIGS. 5a and 5b) that is vertical at zero voltage, bendssmoothly at the critical current I_(c), and then has a linear I-Vrelationship (constant resistance). The V-Φ characteristic of an idealSQUID (compare FIGS. 6a and 6b) is smooth and approximately sinusoidal,with each period of the sinusoid corresponding to the application of asingle flux quantum to the SQUID.

The I-V characteristic of the prior art SQUID sensor of FIG. 1, seen inFIG. 5a shows substantial deviations from ideality. Its complexstructure induces radio frequency resonances that show up as steps inthe nominally linear portion of the I-V curve. In contrast, the I-Vcharacteristic of the improved SQUID sensor of FIG. 2 (FIG. 5b) is muchmore ideal.

FIG. 6a shows the V-Φ characteristic of the prior art SQUID sensor ofFIG. 1. Here the deviations from sinusoidal behavior are quiteremarkable. FIG. 6b shows the V-Φ characteristic of the improved SQUIDsensor of FIG. 2. The characteristic is substantially sinusoidal over awide range of SQUID bias currents.

Conclusion

Thus the reader can see that the improved coupling scheme for SQUIDsensors allows coupling of the feedback loop to the SQUID loop by mutualinductance while restricting the coupling of the SQUID to externalmagnetic fields and confining the effect of the feedback loop to theSQUID.

While the above description contains many specific details, these shouldnot be construed as limitations on the scope of the invention, butrather as an exemplification of one preferred embodiment of it. Manyother variations are possible. For example, a single junction rf SQUIDmight be preferred in some applications. In other applications it mightbe desirable to add damping resistors in parallel with the feedbackloop, SQUID loop or Josephson junctions, or between the ground plane andthe other layers. Other variations will no doubt occur to those skilledin the art of superconductive electronics.

Additional advantages and modifications will no doubt be found by thoseusing the above-described SQUID coupling topology. Therefore, the scopeof the invention should be determined not by the embodiment illustrated,but by the appended claims and their legal equivalents.

What is claimed is:
 1. A magnetic sensor comprising:a superconductingground plane; a SQUID comprising a pickup loop, a SQUID loop and atleast one weak-link junction, said SQUID loop and said pickup loop lyingin a single plane, said SQUID loop being displaced from saidsuperconducting ground plane in a direction normal to the plane andsubstantially overlying said superconducting ground plane and saidpick-up loop lying exterior to the area overlying said superconductinggroundplane; and a feedback loop, said feedback loop being displacedfrom said superconducting ground plane and from said SQUID in adirection normal to the plane, said feedback loop substantiallyoverlying said SQUID loop, and said feedback loop having a bifilargeometry and a mutual inductance with said SQUID loop wherein saidfeedback loop conducts a current in a direction around the feedbackloop, said current producing a magnetic field at each point along saidfeedback loop, and the sum of said magnetic fields being approximatelyequal at a distance from said feedback loop.
 2. The magnetic sensor ofclaim 1 wherein said feedback loop is intermediate said SQUID and saidsuperconducting ground plane.
 3. The magnetic sensor of claim 1 whereinsaid SQUID is intermediate said feedback loop and said superconductingground plane.
 4. The magnetic sensor of claim 1 further comprising asecond superconducting ground plane, said second superconducting groundplane being displaced from said SQUID, said feedback loop, and saidsuperconducting ground plane in a direction normal to the plane, andsaid SQUID and said feedback loop lying between the two superconductingground planes.
 5. The magnetic sensor of claim 1 further comprising anelectrical connection between said SQUID and said superconducting groundplane.
 6. The magnetic sensor of claim 2 further comprising a secondsuperconducting ground plane, said second superconducting ground planebeing displaced from said SQUID, said feedback loop, and saidsuperconducting ground plane in a direction normal to the plane, andsaid SQUID and said feedback loop lying between the two superconductingground planes.
 7. The magnetic sensor of claim 3 further comprising asecond superconducting ground plane, said second superconducting groundplane being displaced from said SQUID, said feedback loop, and saidsuperconducting ground plane in a direction normal to the plane, andsaid SQUID and said feedback loop lying between the two superconductingground planes.
 8. The magnetic sensor of claim 2 further comprising anelectrical connection between said SQUID and said superconducting groundplane.
 9. The magnetic sensor of claim 3 further comprising anelectrical connection between said SQUID and said superconducting groundplane.